Building Bulletproof Transaction Processing in Fintech

When handling real money, there's zero tolerance for errors. Here's how we architected a financial transaction processing system that handles savings, loans, and investments reliably.

Core Requirements

Financial systems have unique constraints:

• Atomicity: Operations must complete fully or not at all
• Idempotency: Duplicate requests must produce the same result
• Audit Trail: Every transaction must be traceable
• Race Condition Prevention: Concurrent operations can't corrupt state

Architectural Patterns

Database Design

We used PostgreSQL with these patterns:

• Immutable Ledger: All transactions are append-only
• Double-Entry Accounting: Every debit has a corresponding credit
• Optimistic Locking: Version numbers prevent lost updates
• Serializable Isolation: Highest transaction isolation level for critical operations

Idempotency Keys

Every API request includes an idempotency key:

interface TransactionRequest {
  idempotencyKey: string;
  amount: number;
  fromAccount: string;
  toAccount: string;
}

We store these keys with transaction results, ensuring duplicate requests return the same response without re-processing.

Scheduled Operations

BullMQ handles time-based financial operations:

• Automated loan repayments
• Interest calculations
• Recurring investment contributions

Jobs are designed to be safely retryable with at-least-once delivery semantics.

Testing Strategy

Financial code requires extensive testing:

• Unit tests for business logic
• Integration tests with real database transactions
• Property-based tests for invariants
• Load tests simulating concurrent operations

Lessons Learned

1. Design for failure: Assume every operation can fail and handle it gracefully
2. Audit everything: Comprehensive logging saved us multiple times during debugging
3. Test concurrency: Race conditions only appear under load
4. Keep it simple: Complex code is harder to verify and trust

Building fintech systems is challenging, but following these patterns helps create systems you can trust with real money.